1. Field of the Invention
This invention relates to the field of electrical switches and particularly to the field of remotely controlled electrical switches capable of interrupting the application of a source voltage to a load.
This invention relates more particularly to the field of solid state power controllers for dc (direct current) service or solid state electrical switches such as solid state relays for use in aircraft applications. Applications for the invention solid state dc switch include marine as well as aircraft and industrial applications that require noise-free closure and interruption of dc source power, compact size, extended switch life, low power dissipation, immunity to high vibration, mechanical shock and high reliability.
2. Description of the Prior Art
Electromechanical switching devices, such as relays, using solenoid-driven means to transfer electromechanical contacts to apply a source voltage to a load are well known. When coupled with an electromechanical circuit breaker, a relay provides a remotely controllable power control function capable of supplying ac or dc service to load via its closed contacts in series with an electromechanical circuit breaker. Electromechanical switching devices, such as relays, provide a very low voltage drop at the switch closure, thereby affording low power dissipation.
As used in dc power control applications, mechanical contacts cannot apply or remove power to a load free of bounce and arcing. Mechanical wear, electrical arcing and slow response also limit the application of electromechanical devices to applications requiring low closing cycle rates.
The solid state dc SWITCH is known to overcome the problem of contact bounce in applying a source voltage to a load. Solid state dc switches are also able to interrupt load current. However, solid state dc switches introduce a substantial voltage drop at the point of circuit closure, thereby providing relatively high power dissipation in comparison to electromechanical switching devices. Solid state dc switches find their greatest application in controlling electrical service to loads requiring relatively low load currents.
Environmental effects such as vibration, moisture, temperature and age, along with operational effects such as thermal and mechanical fatigue from past overload cycle experiences, contribute to the degradation of electromechanical switching of these assemblies. Contact arcing that is sustained by a high voltage source as the contacts separate contributes to electromechanical switch contact degradation, and produces transient noise disturbances that can contribute to error logic signal generation. In addition, mechanical switches provide no means for controlling the rate of rise of voltage on the load as the switch is closed. The closure of a mechanical switch in applying a dc source voltage to a load or to the service feeding a load can produce dv/dt rates of millions of volts per second. High dv/dt rates results in coupling noise spikes capacitively through the insulation of switched service, thereby interfering with other on-board electronics. The negative effect of the high voltage switch drop of a solid state dc switch is diminished by elimination of electromechanical switch contact degradation and transient noise disturbance in high voltage applications such as the 270 Vdc service proposed for avionics equipment in the 1990s.